1. Field of the Invention
The present invention relates to a crawler tractor used for agricultural work and the like. More particularly, it relates to a driving mechanism, an HST and a brake mechanism concerning speed change, traveling and steering of the crawler tractor.
2. Related Art
Conventionally, there is a well-known agricultural working vehicle (crawler tractor) provided with a hydraulic stepless transmission for traveling and a hydraulic stepless transmission for turning as one typical hydraulic-driving agricultural working machine having left and right independent crawler type traveling devices.
It is important for improving operativity of the crawler tractor to ensure transmission/isolation of driving force and braking. Therefore, a crawler tractor is provided with a neutral-position holding mechanism for ensuring neutral location of the HST for traveling and the HST for turning.
As described in the Japanese Patent Laid Open Gazette Hei. 8-242601, for enhancing certainty of cutting off transmission of driving force to crawler traveling devices at the time of stopping the vehicle, with respect to some crawler tractors, an additional clutch device is disposed on a driving power train and a hydraulic circuit is provided for controlling the clutches. With respect to other crawler tractors, a neutral return mechanism for a steering device is disposed in the vicinity of the HST for turning and a steering shaft connected to the steering device is connected to the HST for turning through an universal joint or the like so as to transmit steering operation.
With regard to the above-mentioned hydraulic circuit, there is a crawler tractor constructed such that an oil cooler for cooling pressure oil supplied into a transmission casing is disposed in front of a radiator and the oil cooler is cooled by cooling wind generated from a fan driven by an engine. Also, as shown in the Japanese Patent Laid Open Gazette Hei. 10-54462, there is a crawler tractor such that pressure oil supplied from a charge pump is cooled by an oil cooler and then the fluid is supplied to each HST.
As disclosed in the Japanese Patent Laid Open Gazette Hei. 8-280206, for example, there is a well-known construction for connecting a sub control valve for controlling a working machine connected to a crawler tractor, wherein the sub control valve is detachable, and when the sub control valve is attached, hydraulic ports are connected with respective ports of the sub control valve through hydraulic pipings, and when the sub control valve is not attached, metal is disposed so as to cover an upper surface of a cylinder casing and the ports are connected mutually through oil passages provided within the metal.
However, with regard to many of the conventional crawler tractors, an inscribed gear is used as one of gears constituting a planetary gearing of a differential mechanism for independent left and right crawler type traveling devices. Accordingly, the planetary gearing becomes diametrically large.
Besides, for providing a steering brake to the left and right axles, left and right brake mechanisms are required, and they are large-sized so as to brake a large torque. Such a mechanism results in not only restriction of design of the crawler tractor but also increase of full length of a linkage, thereby hindering improvement of operativity at the time of braking.
Furthermore, conventional mechanisms for holding the HST for traveling and the HST for turning in neutral and a conventional mechanism for holding a steering device in neutral are complicated in construction and require much effort for assembling thereof and the like, thereby hindering reduction of costs for their production.
Besides, a conventional main circuit of the HST for traveling is not provided with a valve for releasing pressure from the HST for traveling. Accordingly, pressure is accumulated in the HST for traveling, thereby making an output shaft hard to rotate, requiring a large power for starting thereof, and hindering smooth change of speed or gears.
With regard to cooling of pressure oil, if an oil cooler for cooling pressure oil used for a transmission is disposed alongside of a capacitor for an air-conditioner and a radiator of an engine, cooling efficiency may fall. Furthermore, maintenance work may become complicated.
With regard to the conventional construction of hydraulic piping, it takes a long time for attaching the piping. Every when the oil pressure is selectively extracted to either front or rear side, it is necessary to close hydraulic ports on an unused side.